This invention relates to single and multi-core electric cables and is particularly concerned with cores for such electric cables having an extruded dielectric of inuslating material to the outer surface of which is applied a conductive or semi-conductive layer constituting a dielectric screen. Examples of insulating materials of which the core dielectric may be made include natural rubber compositions, synthetic rubber-like compositions such as butyl rubber, ethylene/propylene rubber, silicone rubber and styrene butadiene rubber and synthetic plastics materials such as olefin polymers, for example ordinary or cross-linked polyethylene, and olefin copolymers, for example of ethylene and propylene, all such materials hereinafter for convenience being included in the generic expression "rubber or plastics material".
In order to prevent the formation of spaces between a core dielectric and the dielectric screen in which electrical discharges can take place it is desirable that the dielectric screen should fit tightly on the core dielectric and with this purpose in view it has been proposed to form the dielectric screen of a cable core having an extruded dielectric of rubber or plastics material by extruding about the core dielectric a layer of rubber or plastics material rendered electrically conductive by the incorporation of suitable additives, the extrusion of the dielectric screen being effected in such a way and the materials being such that, throughout its length, the extruded dielectric screen bonds firmly to the core dielectric. Whilst such a firm bond is highly desirable from an electrical point of view it has the disadvantage that, when jointing or terminating a cable incorporating a screened core of this form, it is difficult to cut back and strip the extruded dielectric screen cleanly from the core dielectric.
It is an object of the present invention to provide an improved cable core having a dielectric screen which can be easily removed for jointing or terminating purposes.